I am working on a 1983 Camaro Z28 equipped with the 5.0L engine and 5-speed T5 transmission. The car has a limited slip diff with 3.73 gears. The problem is a road speed vibration that seems to come in strongest from about 50 MPH to 70 MPH that seems almost "harmonic"...that is to say the steering wheel is smooth, you don't feel it in the seat of your pants, and it is there even if you shut the engine off at speed. It seems to be higher in the car although you can feel some vibration in the shifter. Even more weird is that if you push the clutch in, the vibration is more pronounced. The car recently had a 4-wheel brake job (front disc/rear drum) complete with all hardware at a reputable shop and the work has been double checked at another shop as being okay. The vibration MAY have appeared after the brake job, but due to an ownership change, I'm not sure.
Here is what has been done so far since the car was brought in with the problem. Keep in mind these things were done in succession, not all at once.
1) The tires are in "like new" condition and have been balanced and were checked for defects and runout in the aluminum wheels...all were okay.
2) There have been three different balanced driveshafts installed, all with new universal joints and different slip yokes...none of which affected the vibration at all.
3) The rear differential was disassembled and checked, the bearings replaced even though they appeared okay, ring & pinion checked to specs, axles checked for flange runout, axle bearings checked for wear and play, and new oil and GM posi additive installed on re-assembly...no affect at all.
4) The clutch was replaced with a new complete unit (disc, pressure plate, release bearing, pilot bushing), the release fork and pivot stud were replaced and the flywheel was resurfaced...no improvement.
5) The transmission was removed and disassembled and inspected for wear and was in excellent condition (better than most T-5's). A complete bearing kit was installed anyway, the input shaft was replaced due to a slight amount of wear on the pilot, the output shaft was checked on a lathe for runout and straightness, the tailhousing bushing was replaced and new Dexron III installed upon reassembly...no difference.
6) The transmission mount was replaced with a new unit, the torque arm mount was replaced with an Energy Suspension urethane unit, the torque arm bolts on the rear diff were removed and reinstalled to proper torque specs...no improvement.
7) The exhaust system was checked for clearance on all components, checked for restrictions or any defects...all okay.
8) The brakes were checked again, just because I was running out of ideas, and they all seem to be okay.
The car still has the vibration and it still is worse when the clutch is depressed. Again, the vibration does not change at all if you shut the engine off on the highway while it is vibrating. I am really looking for some help here...I would like to own another Camaro, but not because I can't fix it!!!!HELP!!!!!!!

Do you still feel the vibration in the shifter with the clutch depressed? 1st thing I would do is find a shop that can check the balance of the tires without removing them...there is a device that spins the tires, brakes, etc all at once while the tires are on the car. This might point you in the right direction.

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375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova

Make sure the pinion angle is correct. It almost sounds like something is moving in the rear to change the angle. If you shut off the engine and it gets worse that tells me that when you unload the drive shaft something is moving in the rear to change the pinion angle. Take a real good look at mounts or the unibody itself. Tire balance etc will not be affected by engine speed just wheel speed. I wish you luck

You mentioned that the wheels were checked for runout - how about the tires? This sounds like "smooth road shake", which results from either wheels or tires (or both) being slightly out of round. The industry recognized this in the late 70's, and had the wheel and tire manufacturers do a runout test on every wheel and tire; the "low point" on the wheel was marked with wax or a sticker, and the same was done for the "high point" of the tire; when they were mounted at the assembly plants, the marks on the tire and wheel were aligned so the high and low points cancelled each other out. You'll see this today on every new car tire and wheel before the prep guy removes them at the dealer.

I'd mark the tires and wheels (so you know where you started), have a tire shop spin them, check the tread surface for runout, and when you see it, break the tire down and rotate it 180 degrees and do it again until the runout is reduced.

Aftermarket tires are NOT round, and neither are wheels; they need to be high point/low point matched, just like the assembly plants do - there's a reason the OEM's spend the extra money for tires, wheels, and assembly labor to match them up.

Give it a try - you've eliminated the other possibilities, and the pinion angle is fixed by the torque arm.

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JohnZ
'69 Z28 Fathom Green

If it is a tire problem why would unloading the drive train make it worse. Tire problems change with road conditions or speed. If I am not mistaken the torque arm only locates one half of the rear end how about the attachments at the other half.

what happens if you maintain speed while depressing lightly on the brake? im with johnz on this one it sounds like balance/wheel/tire problem have you tried switching the wheels around?

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chris
1980 Z/28
(under construction)